1. Technical Field
The present invention relates generally to microphones and particularly to methods and apparatus for enhancing directional capabilities of microphone systems. The invention has particular utility in small microphone applications involving focused sound reception in noisy environments, such as hearing-assistive devices worn by hearing-impaired individuals, voice-controlled computers, and the like.
2. Discussion of the Prior Art
One aspect of the present invention relates to the use of first order bidirectional gradient microphones in communication applications where undesired background noise is present. Another aspect of the invention relates to the use of oppositely directed cardioid microphones mounted together for those same applications. Of particular interest are those applications where small size is required, as is the case for wearable devices for the hearing impaired, and for individuals working in noisy areas where noise reduction cups and wearable amplification systems are commonly used. Also of particular interest are applications such as speech responsive computer systems and applications wherein binaural aiding retains or enhances the ability to identify spatial location of sounds by virtue of different intensities appearing at each aided ear.
The microphone systems of the present invention are improvements over the first and second order unidirectional gradient microphones used in the prior art to obtain noise reduction and high forward gain. Although the goals of noise reduction and high forward gain are similar to the goals in using prior art directional microphone types (generally categorized as wave types, such as "shotgun" microphones, combination line and surface microphones, and combination line and cardioid arrays) to obtain high forward gain and noise reduction, the present invention permits realization of small wearable microphone systems as compared to prior art systems that are large and not generally applicable in situations where small size is a requirement.
The ability to comprehend speech and other desired sound signals in the presence of interfering noise signals is invariably degraded as compared to listening under quiet conditions. The degree of degradation is strongly influenced by the signal-to-noise ratio, by the spectral relationship between the desired and the interfering signals, and by the state of the listener's hearing apparatus. An individual with a damaged hearing system has a much more difficult task than an individual with normal hearing; however, in either case, as the signal-to-noise ratio becomes worse, so does comprehension. All attempts to help a listener under noisy ambient conditions must focus on two considerations. The first is the need to improve, by whatever means, the signal-to-noise ratio for the listener. The second, which is less apparent and not applicable in all situations, is the desirability of avoiding interference with the individual's binaural hearing. Several investigations have shown that binaural hearing improves comprehension under noisy conditions by almost 4 db, a significant amount. While in some situations the problem can be solved by placing a microphone nearer the message source, this is by no means possible in all cases. In the remaining cases, the major strategy is to usually employ some form of directional microphone. For wearable systems, including devices such as hearing aids and other body worn assistive listening systems, the size of the directional microphone is of great significance; because of this, in almost all cases, a type of microphone termed directional gradient is characteristically used.
Directional gradient microphones are a class of microphones that obtains directional properties by measuring the pressure gradient between two points in space. This is in contradistinction to omnidirectional microphones that measure a soundwave produced pressure change referenced to a closed volume of air and hence have no directional characteristics. For most modern directional pressure gradient microphones, the pressure differential across a single membrane is sensed, the membrane being used to divide a tube into two parts with both ends of the tube left open to receive the pressure signal from an external sound source. For this kind of geometry the pressure gradient appearing across the membrane is a combined function of the tube length on either side of the membrane, any acoustic phase-shifting mechanisms that may be included in either side of the tubing, and the direction of arrival of the sound pressure signal with respect to the orientation of the tube. The most common material used for the membrane in modern microphones is so-called "electret" film that responds to flexure by producing an electrical voltage across its two faces. Microphone assemblies employing one such element are referred to as "first order" microphones; assemblies employing two such elements are referred to as "second order" arrays; and so on. Higher order arrays are generally found to have greater directivity than lower order arrays, but also have other properties that may not be desirable. These include greater susceptibility to wind noise, greater susceptibility to case contact noise, greater bulk and sharper fall-off in gain at low frequencies. Regarding this last point, all first order directional microphones experience a gain decrease of 6 db per octave as the frequency lowers, second order directional microphones experience a 12 db per octave gain decrease as the frequency lowers, and so on.
Pressure gradient directional microphones of whatever order are further divided into two classes depending on whether they are: "unidirectional", having their greatest gain in one direction, usually taken to be along the 0.degree.-axis as depicted in polar plots of microphone gain; or "bidirectional", having their greatest gain in two directions, usually taken to be along the 0.degree.-axis and the 180.degree.-axis. It is worthwhile noting that in neither case is the beam pattern only along the major axis; rather, all of these microphones receive some energy from all directions. However, the maximum reception of energy is along the axis directions as described above, and reception of energy is reduced in all other directions. As examples, the most common type of unidirectional microphone, the cardioid, has a gain of unity at 0.degree., -6 db at +/-90.degree. and -20 db or less at 180.degree.. In contrast, a symmetric bidirectional microphone has a gain of unity at 0.degree. and 180.degree., a gain of -6 db at both +/-45.degree. and +/-135.degree., and a gain of -20 db or less at +/-90.degree.. From this information it is clear that while a unidirectional gradient microphone receives most of its energy from one direction, a bidirectional gradient microphone receives most of its energy from two directions 180.degree. displaced from one another.
An important measure for predicting the performance of various microphone configurations in the presence of noise is the noise-to-signal response. In essence, this is the ratio between the response of the microphone to a uniform noise field and its response to a signal along the direction of its maximum response. For reference, this ratio is taken as unity for an omnidirectional microphone measured under the same conditions. Typical values of this parameter for pressure gradient directional microphones are: 1/3 for first order cardioid elements and 1/12 for second order pressure gradient arrays. A symmetric bidirectional first order pressure gradient microphone typically has a noise-to-signal ratio of about 1/3. In terms of improved signal to noise ratios, these amount to approximately 4.7 db for cardioids, approximately 10.8 db for second order gradient arrays and approximately 4.7 db for bidirectional first order arrays.
In view of the foregoing, it is not surprising that, in applications requiring noise reduction, the selection of microphone pattern is an important consideration. Generally, if circumstances permit, the higher order arrays are used to reduce background noise. In situations where size, cost or other factors limit the applicability of higher order arrays, unidirectional cardioid elements are selected over omnidirectional designs. Bidirectional arrays are seldom employed except in a few special cases. The major reason for not choosing bidirectional microphones is because undesired signals typically appear both in front of and behind the microphone, not merely off to the sides.
Factors included in microphone selection that might mitigate against the use of higher order arrays include: size (higher order arrays are larger than first order arrays); sensitivity to wind noise and case noise (any signals reaching the arrays and not meeting the necessary phase requirements result in large unwanted transient outputs); low output level at low frequencies (as noted previously, second order arrays have decreasing gain at -12 db/octave as frequency decreases); and increased complexity of the accompanying electronics.
In understanding the present invention it is important to appreciate the effects of sound-shadows as may be occasioned by the presence of an object between a microphone element and a given sound source. If the size of the object is larger than the wavelength of the frequencies contained in the sound signal, there is a significant decrease in the energy level arriving at the microphone element. This loss of energy can be very large and generally is more evident at high frequencies because low frequencies have longer wavelengths than high frequencies. For example, a 1000 Hz signal has a wavelength of about one foot while at 100 Hz the wavelength is about ten feet. For the case where the wavelength is long compared to the dimensions of the blocking object, diffraction around the object occurs, resulting in a phase shift of arriving signals but no effective attenuation. Hence, for a hearing aid with a microphone mounted in the ear, high frequency sounds arriving at the microphone site are attenuated if their wavelengths are shorter than the size of the wearer's intervening head, but lower frequencies with longer wavelengths will not be so attenuated. This factor is very important both from a functional point of view (sound directionality in either the aided or unaided ear is mainly determined by high frequency signals being differently attenuated at the two ears , and technically in the selection of an appropriate microphone type for various applications.
In many wearable microphone applications, such as in hearing aids, omnidirectional microphones are used instead of cardioid elements even though it would appear at first blush that the cardioid type would be a better selection since hearing impaired individuals have greater than normal problems with understanding speech in noisy environments. The major reasons for not selecting cardioid microphones, however are that: improvements in signal-to-noise ratio found in actual use are seldom as great as those predicted by laboratory measurement; increases in size and complexity of the hearing aid structure required by the use of cardioid microphones ar often not perceived to be justified by the potential gains in signal-to-noise ratios; and the beneficial effects of head shadow (blocking of sound) in improving signal-to-noise ratio make the realizable difference between the use of omnidirectional elements and cardioid elements very small, usually on the order of 2 db or less which is barely perceivable.
Since bidirectional elements receive as much signal from the rear as from the front (or nearly so, depending on design parameters), these microphone types are never used in wearable microphone applications. When all of the factors affecting noise reduction, including head shadow, are taken into account, the net effect of using bidirectional elements in hearing aids has been considered to be undesirable as compared to either omnidirectional or cardioid microphones. In particular, since most hearing aids are ear-level mounted, the orientation of bidirectional microphones is limited to having the microphone facing forward and backward, meaning that sound energy in the rear is as strongly received as sound energy from the front. It is evident that this is not a desirable mode of operation. Hence, the major application of bidirectional microphones is in controlled situations where it is possible to assure that no sound sources are along the 180.degree. axis. An example of such a use is in a recording or broadcast studio where the location of all sound sources can be controlled.
A further use of directional microphones is in the control of computers where the controlling input signal is a closed vocabulary speech signal. The general method, sometimes referred to as a "speech mouse", is based on speech recognition where the user trains an interface to recognize his voice for a set of commands. A problem commonly encountered in these systems is that the typical office environment is noisy while the recognition circuits require a good signal-to-noise ratio in order to have error free responses. Clearly, the selection of a proper microphone is critical. A further limiting factor is that the cost of these voice response systems are modest, generally well under $1000, and the cost for the microphone must be kept correspondingly low. At present the choices made for the microphone pattern types are usually either cardioids or super-cardioids (both first order gradient types) or, in some cases, second order gradient types. The latter choice results in greater expense and more complicated electronics.
A further related background topic of interest in the use of microphones for communication purposes is how stereo binaural hearing is attained. Normal binaural hearing, with its spatial separation of sound events due to the manner in which sound signals arrive at the ears, permits a listener to distinguish among competing sound events. A major cue used by the human hearing system is the intensity of the sound at each ear. The head sound shadow, taken in conjunction with the location and shape of the external ear, results in considerable difference in sound intensities at the two ears depending on the orientation of the listener's head with respect to the arriving sound signal. For signals above about 1000 Hz, the difference in intensity can be as great as 10 db, depending on the angle of arrival. When binaural aided hearing is implemented in a hearing impaired person with ear-level hearing aids (e.g., behind the ear or in the ear), spatial separation of sound is retained because the microphones are located in the same positions as the ears. This is true, whether omnidirectional or unidirectional microphones are used, because of the effects of head shadow. When the microphones are located on the chest (as in body type hearing aids or in other so-called "assistive listening devices"), the stereo effects are lost even if two cardioid microphones are used. The reason for this is that the change in gain in cardioid microphones, as a function of angle of arrival of the sound signals, is too small to replicate the desirable effects of signal attenuation caused by head shadow. While second order or higher order directional microphones can provide these effects, they are too large, too prone to wind and case noise, have excessive loss of gain at low frequencies and require too complicated electronics to be practical. The result is that, for body type hearing aids and for body worn assistive listening devices, the stereo effect is lost. This is unfortunate because, in addition to a good signal-to-noise ratio, the ability to perceive the direction of arriving sound source is an important second factor in effective hearing in noisy situations. Binaurality also plays an important role in monitoring the sound environment for safety. For example, it is clearly desirable for an individual to be able to use directional perception of tire noise or the like to determine the direction of an approaching vehicle. These issues are of particular importance for a blind individual employing spatial hearing abilities for purposes of navigation.